Liquefied petroleum gas (LPG) is a liquefied petroleum-based or natural gas-based hydrocarbon which is gaseous at ambient temperature under atmospheric pressure by compression while optionally cooling, the main component being propane or butane. LPG is advantageously stored or transported in liquid form. Thus, in contrast with a natural gas that requires a pipeline for supply, it can be filled in a container to be supplied to any place. For this reason, LPG comprising propane as a main component, i.e., propane gas has been widely used as a fuel for household and business use. At present, propane gas is supplied to about 25 million households (more than 50% of the total households) in Japan. In addition to household and business use, LPG is used as a fuel for a portable product such as a portable gas burner and a disposable lighter (mainly, butane gas), an industrial fuel and an automobile fuel.
Conventionally, LPG has been produced by 1) collection from a wet natural gas, 2) collection from a stabilization (vapor-pressure regulating) process of crude petroleum, 3) separation and extraction of a product in, for example, a petroleum refining process, or the like.
LPG, in particular propane gas used as a household/business fuel, can be expected to be in great demand in the future. Thus, it may be very useful to establish an industrially practicable and new process for producing LPG.
As a process for producing LPG, Japanese Patent Laid-open Publication No. 61-23688 discloses that a synthesis gas consisting of hydrogen and carbon monoxide is reacted in the presence of a mixed catalyst obtained by physically mixing a methanol synthesis catalyst such as a Cu—Zn-based catalyst, a Cr—Zn-based catalyst and a Pd-based catalyst, specifically a CuO—ZnO—Al2O3 catalyst or a Pd/SiO2 catalyst with a methanol conversion catalyst composed of a zeolite having an average pore size of about 10 Å (1 nm) or more, specifically a Y-type zeolite, to give a liquefied petroleum gas or a mixture of hydrocarbons similar in composition to LPG.
In terms of the zeolite catalyst, the above-mentioned Japanese Patent Laid-open Publication No. 61-23688 describes that the distribution of hydrocarbons produced significantly depends on the pore size of the zeolite, and that when a zeolite having a larger pore size (Y-type zeolite) is used, lower paraffins of C1 to C6, particularly C2 to C4, are produced with a high selectivity while controlling production of aromatic hydrocarbons. In terms of the zeolite catalyst, the above-mentioned Japanese Patent Laid-open Publication No. 61-23688 also describes that although it has a restriction for a pore size, any type of zeolite catalyst meeting the condition can be applied, irrespective of variation in a molecular structure or micropore structure or a variety of preparation treatments. Meanwhile, in terms of a methanol synthesis catalyst, the above-mentioned Japanese Patent Laid-open Publication No. 61-23688 describes that a variety of simple substances and composites of a metal or a metal-oxide can be used if they have the ability to hydrogenate.
Furthermore, the above-mentioned Japanese Patent Laid-open Publication No. 61-23688 also describes that acidity of a zeolite can be increased by dealumination, and thereby the yield of hydrocarbons (lower paraffins) can be improved, from the comparison between the reaction results using a mixed catalyst comprising a dealuminated Y-type zeolite with SiO2/Al2O3=7.6 as a methanol conversion catalyst and the reaction results using a mixed catalyst comprising a non-dealuminated Y-type zeolite with SiO2/Al2O3=5.1.
The catalyst described in the above-mentioned Japanese Patent Laid-open Publication No. 61-23688, however, does not necessarily show sufficient performance.
For example, a catalyst consisting of Pd/SiO2 and a Y-type zeolite is less active and gives a lower yield of a hydrocarbon, in which a ratio of propane (C3) and butane (C4) is lower. A catalyst consisting of Pd/SiO2 and a dealuminated Y-type zeolite with SiO2/Al2O3=7.6 treated with steam at 450° C. for 2 hours has a higher activity and gives a higher yield of a hydrocarbon, in which a ratio of propane (C3) and butane (C4) is higher, but it may not have sufficient performance particularly in terms of activity and yield of a hydrocarbon. Moreover, in general, a catalyst consisting of Pd/SiO2 and a Y-type zeolite may be significantly deteriorated with time, and thus, it may not have a sufficiently long catalyst life.
On the other hand, as a general trend, a catalyst consisting of a Cu—Zn-based catalyst (a copper-zinc-alumina mixed oxide and a commercially available catalyst for methanol synthesis at a low pressure) and a Y-type zeolite has a higher activity and gives a higher yield of a hydrocarbon, in which a ratio of propane (C3) and butane (C4) is higher, in comparison with a catalyst consisting of Pd/SiO2 and a Y-type zeolite. Among them, a catalyst consisting of a Cu—Zn-based catalyst and a dealuminated Y-type zeolite with SiO2/Al2O3=7.6 treated with steam at 450° C. for 2 hours has a high activity and gives a high yield of a hydrocarbon, in which a ratio of propane (C3) and butane (C4) is high. However, a catalyst consisting of a Cu—Zn-based catalyst and a Y-type zeolite may be significantly deteriorated with time, and thus, it may not have a sufficiently long catalyst life in general. It is, therefore, difficult to stably produce LPG in a high yield for a long period using the catalyst.
Thus, it has been needed further improvement of a catalyst for producing a liquefied petroleum gas in order to practically use a process for producing LPG from a synthesis gas and a process for producing LPG from a carbon-containing starting material such as a natural gas.
As a process for producing LPG, “Selective Synthesis of LPG from Synthesis Gas”, Kaoru Fujimoto et al., Bull. Chem. Soc. Jpn., 58, p. 3059-3060 (1985) discloses that a hybrid catalyst consisting of a methanol synthesis catalyst such as a 4 wt % Pd/SiO2, a Cu—Zn—Al mixed oxide {Cu:Zn:Al=40:23:37 (atomic ratio)} or a Cu-based low-pressure methanol synthesis catalyst (Trade name: BASF S3-85) and a high-silica Y-type zeolite with SiO2/Al2O3=7.6 treated with steam at 450° C. for 1 hour can be used to produce C2 to C4 paraffins in a selectivity of 69 to 85% via methanol and dimethyl ether from a synthesis gas. However, as is for the catalyst described in the above-mentioned Japanese Patent Laid-open Publication No. 61-23688, the catalyst described in the reference may not show sufficient performance.
In addition, “Synthesis of LPG from Synthesis Gas with Hybrid Catalyst”, Qianwen Zhang et al., Dai 33 Kai Sekiyu Sekiyu Kagaku Toronkai Koen Yoshi (the summaries of the 33th Petroleum and Petrochemistry Discussion), p. 179-180, Nov. 17, 2003 discloses that a hybrid catalyst consisting of Pd—SiO2 or Pd, Ca—SiO2 and a zeolite can be used to produce LPG from a synthesis gas.